Computed tomography (CT) systems and methods are widely used, particularly for medical imaging and diagnosis. CT systems generally create images of one or more sectional slices through a subject's body. A radiation source, such as an X-ray tube, irradiates the body from one side. A collimator, generally adjacent to the X-ray source, limits the angular extent of the X-ray beam, so that radiation impinging on the body is substantially confined to a planar region defining a cross-sectional slice of the body. At least one detector (and generally many more than one detector) on the opposite side of the body receives radiation transmitted through the body substantially in the plane of the slice. The attenuation of the radiation that has passed through the body is measured by processing electrical signals received from the detector.
These conventional detectors are called energy-integrating detectors for acquiring energy integration X-ray data. Recently, photon-counting detectors are configured to acquire the spectral nature of the X-ray source rather than the energy integration nature in acquiring data. To obtain the spectral nature of the transmitted X-ray data, the photon-counting detectors split the X-ray beam into its component energies or spectrum bins and count a number of photons in each of the bins. The use of the spectral nature of the X-ray source in CT is often referred to as spectral CT. Since spectral CT involves the detection of transmitted X-rays at two or more energy levels, spectral CT generally includes dual-energy CT by definition.
Semiconductor-based photon-counting detectors used in spectral CT can detect incident photons and measure photon energy for every event. However, due to the interaction depth and ballistic deficit, the measured photon energy cannot be related to incident photon energy uniquely. At high flux, pulse pileup may cause count lose too. Therefore, as recognized by the present inventor, a detector response function for semiconductor-based photon-counting detectors (e.g., CZT or CdTe) is required to describe the count rate nonlinearity and energy response.